AN EXPERIMENTAL ANALYSIS OF THE HYPOTHALAMIC-HYPOPHYSIAL-THYROID RELATIONSHIP IN THE RAT

A novel, nonradioactive in vivo bioassay of thyrotropin (TSH)

A new and simple in vivo bioassay suitable for routine testing of pituitary and recombinant TSH preparations was developed. Male Albino Swiss CF-1 mice were given T3 in their drinking water to suppress endogenous TSH. T3, 3.0 micrograms/mL, given to mice for a period of 4 days decreased plasma total T4 (TT4) level to less than 10% of the nonsuppressed (control) level. Various doses of exogenous pituitary and recombinant TSH preparations were injected intraperitoneally and blood samples were obtained from the orbital sinus 6 h later. The TT4 level, measured by radioimmunoassay, served as the assay end-point. The assay required injection of approximately 3.0 micrograms of pituitary human TSH (phTSH), 1.0 microgram recombinant human TSH (rhTSH), 0.2 microgram bovine TSH (bTSH), and 0.1 microgram rat TSH (rTSH) to attain half-maximal response. The maximal level of TT4 after TSH stimulation was similar to that observed in normal, nonsuppressed mice. The procedure developed is relatively easy to perform, economical, and, unlike earlier TSH bioassays, does not require the use of radionuclides. This bioassay showed acceptable sensitivity and reliability in structure-function studies of pituitary TSH from different species as well as rhTSH.

Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold

It seems clear from the studies reviewed here that there is adequate evidence to support the concept of a biphasic response of the thyroid gland to cold as first postulated by Moll et al. (1972). The initial response to acute exposure to cold begins at the level of the hypothalamus as a result of either neural stimuli from skin and other areas and/or blood of somewhat lower than normal temperature reaching the hypothalamus (Andersson et al., 1963). As a result, the secretion of norepinephrine and/or dopamine may increase, and serotonin and/or somatostatin may decrease. The net result of these is an increase in the release of TRH from the hypothalamus. This, in turn, stimulates the cascade for the release of TSH from the anterior pituitary gland and thyroid hormone from the thyroid gland. Moll et al. (1972) postulated the lack of a feedback limb in this acute phase, and, indeed, this may be the case. It is possible, however, that certain hormones, such as somatostatin, norepinephrine, T3, and T4 could act in the capacity of feedback inhibitors. Additional experiments will be required to assess this possibility. The transitional link between the acute (less than 1 day) and chronic (greater than 1 day) phases of the response of the thyroid gland to cold could be T4 itself. An increase in the concentration of T4 in plasma has been reported to increase peripheral deiodination of T4 to T3 by kidneys and liver of rats. There are no studies at present to indicate that hepatic conjugation can be increased by elevation of plasma levels of T4 and T3. If it can, these responses would provide adequate reasons as to why peripheral metabolism of thyroid hormones increases during chronic exposure to cold. The time-course for these changes to occur needs to be studied in greater detail to establish the sequence of events following acute exposure to cold. The latter may also increase urinary excretion of T4 and T3 in man, but not the rat. This suggests that another aspect of exposure to cold needing additional study is measurement of the binding affinities of T4 and T3 for their transport proteins during exposure to cold as compared to affinities prior to exposure to cold. If binding affinities are reduced, the amount of free hormones would increase and, consequently the likelihood of being excreted into urine and conjugated by the liver would also increase.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothalamic control of thyrotrophin secretion

1. By means of the haemagglutination-inhibition technique, it has been possible to measure the plasma and pituitary levels of thyroid-stimulating hormone (TSH) following unilateral and bilateral electrolytic lesions placed in the supraoptic area of the hypothalamus of female rats.2. Unilateral lesions after 8 and 48 hr caused a temporary decrease in the percentage of circulating TSH followed by a return to normal.3. Bilateral lesions caused a fall in plasma TSH to a level 41% below normal at 10 days whereas the pituitary level increased 70%.4. Rats with bilateral symmetrical supraoptic lesions, kept for 3 days at 4 degrees C, had a lower plasma TSH content (23%) and a higher pituitary TSH content (16%) than the sham operated control animals but the levels of pituitary TSH did not show any significant difference (0.100 > P > 0.050).5. Thyroidectomized rats with bilateral lesions kept at 26 degrees C for 3 days had a much lower plasma TSH (39%) and a slightly higher pituitary TSH as compared to the normal intact animals, suggesting that the hypothalamus also influences the synthesis of TSH in the pituitary.6. Thyroidectomized rats with bilateral lesions kept at 4 degrees C for 3 days showed both a plasma and pituitary TSH increase compared to controls at 26 degrees C, suggesting that when a higher demand for thyroid function is present the pituitary gland has some autonomy for both secretion and release of TSH. This autonomy appears to be slight, as there was no statistically significant difference between the pituitary TSH levels of the thyroidectomized animals bearing similar supraoptic lesions and exposed to 4 and 26 degrees C.